The present invention relates to an objective optical system for endoscopes.
So far, there have been wide-angle optical systems used in consumer-oriented applications such as digital cameras (see Patent Publications 1 to 4), and optical systems used on endoscopes as well (see Patent Publications 5 and 6).
Patent Publication 1: JP(A)2004-109591
Patent Publication 2: JP(A)2004-62014
Patent Publication 3: JP(A)2004-45978
Patent Publication 4: JP(A)2003-195158
Patent Publication 5: JP(A)2-176612
Patent Publication 6: JP(A)2-69710
Patent Publication 7: PCT 99/6866
However, the optical systems set forth in Patent Publications 1 to 4 have not had an angle of view enough to be used on endoscopes, although having a wide-angle arrangement. The optical systems set forth in Patent Publication 5 or 7, albeit used in endoscope applications, have still been less compatible with CCDs having a lot more pixels, because of the occurrence of field curvature. Further, the optical system set forth in Patent Publication 6 has had an angle of view only par with that for consumer-oriented applications.
With such problems in mind, an object of the invention is to provide a small-format yet wide-angle objective optical system for endoscopes.
According to the invention, the aforesaid object is accomplishable by the provision of an objective optical system for endoscopes, which comprises, in order from an object side thereof, a stop, a first lens and a second lens, wherein the first lens comprises a positive meniscus lens concave on its object side and the second lens comprises a positive lens convex on its object side, with the satisfaction of the following conditions (1), (2), (3) and (4):−1.2<r1/f<−0.8  (1)−1.0<r2/f<−0.6  (2)2.1<f2/f<4.2  (3)nd1>1.65  (4)where r1 is the radius of curvature of the object-side surface of the first lens,
r2 is the radius of curvature of the image-side surface of the first lens,
f is the focal length of the whole optical system,
f2 is the focal length of the second lens, and
nd1 is the d-line refractive index of the first lens.
Field curvature is quite well corrected by the satisfaction of conditions (1) to (4). It is then possible to be compatible with CCDs having a lot more pixels.
Condition (1) is concerned with the radius of curvature of the object-side surface of the first lens and the focal length of the whole optical system. As the lower limit of −1.2 to condition (1) is not reached, it causes under-correction of field curvature, rendering an image plane under. As the upper limit of −0.8 to condition (1) is exceeded, it causes overcorrection of field curvature, rendering the image plane over.
Condition (2) is concerned with the radius of curvature of the image-side surface of the first lens and the focal length of the whole optical system. If the range of condition (2) is satisfied, that is preferable because of being capable of making sure large angles of view and high image quality. As the lower limit of −1.0 to condition (2) is not reached, it fails to satisfy any wide-angle requirement. Exceeding the upper limit of −0.6 to condition (2) is not preferable because the image plane becomes minus with an increasing astigmatism. It also takes part in correction of chromatic aberrations, resulting in increases in both longitudinal chromatic aberration and chromatic aberration of magnification. In addition, it is not preferable in that there is a drop of the contrast of images with blurred colors.
Condition (3) is concerned with the focal length of the second lens and the focal length of the whole optical system. Being short of the lower limit of 2.1 to condition (3) causes undercorrection of field curvature, rendering the image plane under. Exceeding the upper limit of 4.2 to condition (3) causes astigmatism to grow large.
Condition (4) is concerned with the d-line refractive index of the first lens. Being short of the lower limit of 1.65 to condition (4) gives rise to overcorrection, rendering the image plane over.
It is also desired to satisfy the following conditions (5), (6), (7) and (8):1.0<r3/f<1.8  (5)3.2<lt/ih<5.2  (6)5<|ν2−ν1|<35  (7)2w>85°  (8)where r3 is the radius of curvature the object-side surface of the second lens,
lt is the total lens length (from the first surface to the imaging plane),
ih is the maximum image height,
ν1 is the Abbe constant of the first lens,
ν2 is the Abbe constant of the second lens, and
2w is the maximum angle of view.
Condition (5) is concerned with the radius of curvature the object-side surface of the second lens and the focal length of the whole optical system. Abiding by the range of condition (5) is preferable because of being capable of making sure a back focus. Being short of the lower limit of 1.0 to condition (5) is not preferable because power grows too strong to take hold of any back focus, rendering the range of focusing narrow. Exceeding the upper limit of 1.8 to condition (5) causes the angle of oblique incidence of light on the CCD to grow tight, encountering difficulty making sure the quantity of rim light.
The optical system is located almost vertically to the direction of insertion of an endoscope's end. That vertical location makes it possible to shorten the total length, and is best fitted for an oblique or side view type objective, dispensing with members for changing the field of view such as a prism.
Condition (6) is concerned with the total lens length and the maximum image height. Exceeding the upper limit of 5.2 to condition (6) causes the total lens length to grow long, rendering layout difficult. Being short of the lower limit of 3.2 gets rid of space for receiving filters or the like, and renders lens fabrication difficult.
Condition (7) is concerned with the Abbe constant of the first lens and the Abbe constant of the second lens. Any deviation from the range of condition (7) gives rise to some considerable separations at the C- and F-lines, rendering color blurring conspicuous. Longitudinal chromatic aberration and chromatic aberration of magnification go worse too.
Condition (8) is concerned with the maximum angle of view. This is the minimum requirement for the angle of view for endoscopes required to have a wide-angle arrangement.
It is further desired to satisfy any one of the following conditions (9), (10) and (11):1.3<f2/f1<2.5  (9)0.37<d1/f1<0.55  (10)0.12<d2/f2<0.24  (11)where f1 is the focal length of the first lens,
d1 is the thickness of the first lens,
d2 is the thickness of the second lens, and
f2 is the focal length of the second lens.
Condition (9) is concerned with the focal length of the first lens and the focal length of the second lens, that is, the power rate between the first lens and the second lens. Being short of the lower limit of 1.3 to condition (9) causes the power of the first lens to become weak and so light rays on the second lens to grow high, resulting in an increase in the size of the whole lens system. Being in excess of the upper limit of 2.5 to condition (9) is not preferable, because the power of the first lens grows relatively strong; the bending of spherical aberrations grows large with an increasing field curvature.
Condition (10) is concerned with the thickness and focal length of the first lens. Being short of the lower limit of 0.37 to condition (10) causes the lens to become thin, encountering difficulty in making sure of an edge thickness and, hence, lens processing. Being in excess of the upper limit of 0.55 to condition (10) causes an increase in the proportion of the first lens relative to the total length, resulting in an increase in the size of the whole optical system.
Condition (11) is concerned with the thickness and focal length of the second lens. Being short of the lower limit of 0.12 to condition (11) causes the lens to become thin, encountering difficulty in making sure of an edge thickness and, hence, lens processing. Being in excess of the upper limit of 0.24 to condition (11) causes an increase in the proportion of the second lens relative to the total length, resulting in an increase in the size of the whole optical system. This is also not preferable in that there is large field curvature produced.
Further, it is desired to satisfy the following conditions (12) and (13):0.34<d2/r3<1.0  (12)0.2<d3/f<0.6  (13)where d2 is the thickness of the second lens,
r3 is the radius of curvature of the object-side surface of the second lens,
d3 is a distance from the second lens to a CCD's cover glass, and
f is the focal length of the whole optical system.
Condition (12) is concerned with the thickness of the second lens and the radius of curvature of the object-side surface of the second lens. Being short of the lower limit of 0.34 to condition (12) causes the lens to become thin, encountering difficulty in making sure of an edge thickness and, hence, lens processing. Being in excess of the upper limit of 1.0 to condition (12) causes an increase in the proportion of the second lens relative to the total length, encountering difficulty in making sure the back focus.
Condition (13) is concerned with the distance between the second lens and the CCD's cover glass with respect to the focal length of the whole optical system. With the inventive lens system, focusing is implemented between the second lens and the CCD's cover glass. Here, as the lower limit of 0.2 to condition (13) is not reached, there is a shortage of focusing space that often causes the lens system to be out of focus at the desired distance. Being in excess of the upper limit of 0.6 to condition (13) is not preferable because the total length grows long. It also causes the angle of oblique incidence to become shallow, giving rise to an increase in the diameter of the second lens.
According to the invention, it is possible to provide an objective optical system for endoscopes, which is capable of oblique view and side view without recourse to any member for changing the field-of-view direction, and of small format yet high performance.
Still other objects and advantages of the invention will be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.